This disclosure relates to gas turbine engines, and more particularly to squeeze film dampers utilized in gas turbine engines.
Squeeze film dampers are widely used in gas turbine engines to minimize rotor deflections. The damper is typically contained in a bearing compartment filled with air. A damper film is supplied by oil at an elevated pressure. The oil within the squeeze film damper provides resistance to rotor whirl as it is pushed around the damper annulus. Eventually the oil flows through the end seals of the damper into the bearing compartment where the oil is scavenged and recirculated through an engine lubrication system.
The squeeze film damper generates a force on the rotor by squeezing a film of oil between two cylindrical cross-sectional regions, the outer region fixed to supports and considered rigid, and the inner region whirling with the rotor. A pressure field is developed as the rotor whirls, resolving the net force acting on the rotor with components aligned with the eccentricity and components parallel and perpendicular to the eccentricity, which enables the forces to be expressed in terms of a squeeze film damper generated stiffness and damping constant, respectively. There is a resultant region of positive pressure with respect to the circumferential mean of the pressure within the damper, and also a region in which the pressure is reduced to below a mean pressure.
In the typical system, the mean pressure, or “steady” pressure, within the damper is typically set by the characteristics of the oil supply system and the leakage of the oil through the seals. As the seals approach ideal seals, i.e. no leakage, the mean pressure approaches the supply pressure. The unsteady part of the pressure, or dynamic pressure amplitude, builds with whirl amplitude. The larger the whirl, the larger the dynamic pressure amplitude becomes. This idealized model works well conceptually until the zero-to-peak amplitude of the dynamic pressure exceeds that of the steady pressure. At this point, the simplified model begins to predict negative pressure over certain regions of the circumference. While negative pressures are conceptually possible in some situations, the prediction of negative pressures typically implies that the oil will either cavitate, or the seals on the ends of the damper will back-flow air from within the bearing compartment into the squeeze film region, or some combination of both. Once air, or typically any gas, is entrained within the squeeze film damper, the effectiveness of the damper, and the ability of analytical models to predict the performance of the damper, becomes compromised.
From a design perspective, it is typically preferable to design squeeze film dampers with sufficient mean pressure such that the dynamic pressure does not result in regions of negative pressure. However, in some cases, it is impractical to provide sufficient supply pressure to achieve the desired damping and stiffness characteristics from the damper.